The invention relates to a magnetic resonance apparatus which includes a gradient device including gradient coils for generating a magnetic gradient field in an imaging volume of the apparatus by means of gradient current pulses, each of which is formed from a series of successive gradient pulse samples, and a control device which is connected to the gradient coils in order to produce the gradient pulse samples. The control device includes a converter for producing an analog signal whose value is specified by a discrete signal to be received by the converter, which analog signal is applied to a power amplifier for producing the gradient pulse samples. The control device also includes processor means for calculating a desired value of a first gradient pulse sample, a first discrete value which approximates the desired value of the first gradient pulse sample, and a desired value of a second gradient pulse sample which succeeds the first gradient pulse sample and is associated with the same gradient current pulse, a second discrete value which approximates the desired value of the second gradient pulse sample, where the successive discrete values constitute the discrete signal to be received by the converter.
An apparatus of this kind is known from the published German patent application No. 197 29 431.
A medical MRI (Magnetic Resonance Imaging) apparatus is used to form images of an object to be examined which is situated in an imaging volume of the apparatus in which a uniform, steady field (a so-called main field) exists. A gradient field which varies (usually linearly) as a function of the location is superposed on the main field so as to indicate, in the region to be imaged, the point (x, y, z) which is to be imaged at a given instant. Each point (x, y, z) in the region to be imaged is then indicated by the instantaneous value of an x gradient field, a y gradient field and a z gradient field. The time-dependent variation of these fields is shaped as a pulse, i.e. the so-called gradient pulse which often has a trapezoidal shape and a duration of the order of magnitude of 1 ms. Said gradient fields are generated by pairs of coils (i.e. one pair for each of the x, y and z co-ordinates), each of which is traversed by associated gradient current pulses.
In the case of digital control of the formation of the gradient pulses in an MRI apparatus, the gradient current pulses generating the pulse-shaped gradient fields are composed of directly successive sub-pulses which will be referred to hereinafter as gradient pulse samples and are produced by a power amplifier which may be constructed as a so-called switched mode H bridge. Such a power amplifier includes four output transistors in a H configuration, each transistor being controlled by an input signal which assumes discrete values only. Such an input signal can be produced by a converter, for example a pulse width converter (PWM converter) which is connected to the output of a processor. The processor in the MRI apparatus which is known from the cited German patent application calculates the desired value of a gradient pulse sample; this desired value is subsequently approximated as well as possible by a discrete value. This process takes place for all successive gradient pulse samples which together constitute the gradient current pulse to be generated.
Such a digital formation of the gradient pulse samples involves the problem that the time resolution required for the processor must be so high that state of the art processors cannot satisfy these requirements or only hardly so; this will be demonstrated by the following numerical example. The maximum clock frequency at which the digital formation of the gradient pulse samples takes place is of the order of magnitude of 25 kHz; this value cannot be chosen to be higher, because the power transistors used (IGBTs) cannot follow a higher switching speed. Thus, this results in a period of at least 40 xcexcs for a gradient pulse sample from the PWM converter. The time integral of the gradient pulse is of importance for the formation of gradient pulses, i.e. the deviation therein (xe2x80x9cthe surface errorxe2x80x9d) may amount to no more than 10 xcexcAs; the surface of such a pulse is typically 10 As, meaning a resolution of 10xe2x88x926. (The desired resolution in the current through the gradient coils is 1 mA for a maximum current of 1000 A, which also means a resolution of 10xe2x88x926.) In order to achieve this resolution with a time discrete composition of the gradient pulse sample, therefore, a smallest unit of time of 10xe2x88x926xc3x9740 xcexcs=40.10xe2x88x9212 s is required. The latter time resolution corresponds to a minimum clock frequency of the processor of 25 GHz; this frequency cannot be achieved by commercially available standard processors.
The digital control of the known MRI apparatus aims to solve the described problem by imparting to the gradient pulse sample the shape of a PWM pulse which is formed as the sum of two PWM pulse sections, a first pulse section being formed by the 9 most significant bits of the 22 bits constituting the digital gradient pulse sample. These 9 most significant bits are applied as one data word to the pulse shaper which forms therefrom a pulse having a duration which corresponds to the value of said 9-bit data word. The 13 remaining, least significant bits also form a data word wherefrom the second pulse section is formed. The second pulse section is added in time to the first pulse section. The PWM pulse thus composed constitutes a gradient pulse sample having the desired exact pulse duration.
This known method of forming a gradient pulse sample having the desired exact pulse duration has the drawback that it requires the use of very accurate components. This is because the time accuracy of the first pulse section may not be less than the smallest time unit of the second pulse section, being of the order of magnitude of 40.10xe2x88x9212 s in the above numerical example. It will be very difficult or even impossible to satisfy this requirement in practice.
It is an object of the invention to provide an MRI apparatus of the kind set forth in which gradient current pulses can be generated with the desired accuracy. To this end, the MRI apparatus according to the invention is characterized in that the processor means are also arranged to calculate the difference between the desired value of the first pulse sample and the first discrete value and to store said difference, and to correct the value of the second gradient pulse sample by means of said difference during its calculation.
For the discretization of the desired value of a gradient pulse sample a number of bits is accepted which is (much) smaller than the number corresponding to the desired resolution. As a result, generally speaking, the value of the discretized gradient pulse sample will deviate from the desired value, but this deviation is known, because both the desired value and the discretized value are known. This known deviation (the difference) is saved until the next gradient pulse sample is to be formed. A desired value is then calculated again. The difference calculated during the formation of the preceding gradient pulse sample is then added to or subtracted from the value then found (depending on the sign of the difference) and the value thus calculated is subsequently discretized again as described above. Small deviations from the desired value thus occur for each gradient pulse sample, but it has been found that the average value of this deviation over the duration of the entire gradient current pulse is so small that it can be ignored.
The converter for producing said analog signal in a preferred embodiment of the invention is formed by a digitally controlled pulse width modulated converter. In combination with an MRI apparatus this embodiment offers the advantage that signals originating from such a converter in principle need be filtered merely by a simple low-pass filter so as to remove the RF part of the spectrum and hence obtain the desired gradient pulse samples.
In a further embodiment of the invention the digitally controlled pulse width modulated converter is formed by a suitably programmed microprocessor. This embodiment offers the advantage that the parameters of the PWM function can be readily changed by way of a usually minor modification of the program of the microprocessor. Such a change concerns, for example a change of the pulse period or the fineness of the time raster, i.e. the magnitude in time of the discretization steps of the converter.